The invention relates to a machine for the continuous manufacture of chipboards, fibreboards or similar products from a raw material containing lignocellulose or cellulose which has been reduced to chips or fibres and mixed with an adhesive curable by pressure and/or heat. The machine comprises a heatable pressure drum and an endless metallic pressure belt, which is guided on deflection .rollers and wraps the pressure drum over a portion of its periphery while forming a compacting line for the material to be compressed. The material is heated to a temperature above the curing temperature of the adhesive, a pressure device being provided to exert onto the pressure belt a predetermined compacting pressure acting over an area according to earlier patent application No. 93810458.5.
Such machines are known for instance from EP 195 128, EP 324 070, DE 25 49 560 or DE 38 00 513. They serve to process waste material available in large quantities, such as wood refuse, sugar cane bagasse, cotton stalks or similar material, to chipboards or fibreboards or similar products for use in the building or furniture industry. For this purpose, the raw material containing lignocellulose or cellulose is reduced to chips or disintegrated to fibres in a cleaned and largely dried form mixed with a suitable adhesive. A a particularly suitable material for the adhesion of cellulose fibres and production of strong chipboards (Medium Density Fibre-Board) are, for instance, copolymers of sodium lignosulphonate, melamine and formaldehyde, which gradually cure at a temperature of about 130.degree. C.
For the production of chipboards or fibreboards, the raw material, mixed with adhesive, is brought on to the belt into the compacting line between the belt and the pressure drum moving synchronously therewith, where they are by the action of pressure and temperature compressed and gradually cured, before they are removed from the pressure drum and cut to the desired board size.
In the known machines a pressure roller is provided at the inlet end, i.e., at the inlet of the raw material into the compression zone, which pressure roller exerts onto the belt a nearly linear pressure force with a compacting. pressure considerably above 100 bar. After this inlet end, the pressure roller follows a zone in which the compacting pressure is produced only by the belt tension and after that follows a further pressure roller which also exerts a nearly linear force. Usually three to four rollers are provided, of which the last roller serves to form the produced boards shortly before the end of the curing process. A disadvantage of this known process is that, due to the sudden pressure loading and following zones of small pressure due to multiple springing back of the fibres, an insufficient strength, an undesirable density profile and insufficient hardness distribution across the thickness of the board result and also an unnecessarily large amount of the adhesive must be used.
A further disadvantage is that in the known machines; is that the pressure drum and the pressure rollers cannot be sufficiently heated to transfer enough heat to the material, from which follows that the belt must be brought to the sufficient temperature by additional heating means.
It is especially disadvantageous in the known machines that large forces can act on the bearings of the pressure drum. This requires mounting of the individual pressure rollers such that the bearing forces of the pressure drum are at least partly compensated, but by adjustment to other operational conditions the equalization of forces would be disturbed. It is further disadvantageous that the pressure rollers deform the pressure drum to such an extent that it is not possible to keep narrow tolerances. In order to keep sagging of the pressure drum as small as possible when high forces act on it, a considerable wall thickness of the pressure drum in the decimeter region is needed, which brings the weight of the pressure drum to more than 100 metric tons. In spite of that the sagging of the pressure drum is still so extensive that the thickness of the produced chipboards across the width unacceptably varies and its shape is bent, i.e. departs from a plane.